Long-range electrostatic interaction in DNA-cationic lipid complexes

Recent X-ray synchrotron studies by Radler et al. have revealed that mixtures of DNA with mixed cationic/neutral liposomes can form stacks of two-dimensional smectics of aligned DNA chains intercalated by lipid bilayers. The DNA chains interact via a long-range repulsion. In this letter we show that i) for this arrangement, the electrostatic interaction between the charged chains is a lower-dimensional analog of the classical Langmuir equation for the interaction between charged plates in salt-free water, with the cationic lipids playing the role of counterions, and ii) that for screening by cationic lipids, we encounter an obligatory two-dimensional variant of Manning condensation.